Skip to main content

Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

See other formats


GLACIATION OF THE YAEUTAT BAY REGION
219
flattish bottom revealed in cross-sections and the straightness of fiord walls, a number of similar, parallel, bottom profiles exist in every fiord.
The first point is well brought out by comparing the submerged contours of northwestern Eussell Fiord with those of Nunatak Fiord and southern Russell Fiord. The bottom profile of the former reveals a persistent southeast slope at the rate of nearly sixty feet to the mile, while the latter do not slope consistently but have marked up and down grades. These might perfectly well be due either to irregular glacial scooping or to moraines below sea level, resting on a simple slope similar to that in northwestern Russell Fiord. With glaciers of this size, differential glacial erosion of two or three hundred feet in a distance of several miles in a longitudinal valley is perfectly possible, as glacial rock basins in various regions testify, but submerged moraines of this height are also possible. It is enough to point out here that the bottom profiles are thus interrupted, as in Nunatak Fiord and lower Russell Fiord, in contrast with regular, uninterrupted, bottom slopes in northwestern Russell Fiord and tie channel east of Haenke Island in Disenchantment Bay, and that we ascribe some such interruptions in grade to glacial scooping, and others to submerged moraines. Imperfectly eroded knobs surrounded by
3250
FIG. 17.   SECTION OF STRATI BAY AND HIDDEN GLAGTCR WITH ITS SUBMERGED HANGING VAILEY. SECTION OF RTJBSELL FIOED.   VERTICAL AND HORIZONTAL SCALDS THE SAME.
CBOSB-
deep water also exist, such as the 12 foot shoal near Osier Island where the Princess Maquinna grounded in 1913.
The second point, regarding longitudinal fiord bottom profiles in contrast with longitudinal profiles of stream valleys, cannot be discussed adequately with the existing doubt regarding the cause of these interruptions of fiord bottom slopes. If some of the two or three hundred foot interruptions in slope are in rock, and not due to moraine accumulations, we have an excellent argument in favor of the glacial origin of these fiords, for upgrades of 200 to 800 feet are impossible in stream valley profiles, whereas rock basins would naturally develop in fiord bottoms or in valleys above sea level when eroded by ice.
The map shows clearly that many parallel longitudinal bottom profiles are possible with the width of the flattish fiord floors, as in northwestern Russell Fiord. This contrasts markedly with the bottom of a submerged, stream-eroded valley where the medial bottom profile is rarely straight and almost never could have a single similar profile parallel to it. There are no submerged spurs entering the fiord from either side. The bottom profiles, longitudinally, show that the fiord is exactly similar in plan above and below water and is characteristically of the large, simple pattern produced jay great ice erosion.
The submerged hanging valley is well illustrated by Seal Bay (Fig. 17) which is from 180 to 600 feet deep and hangs approximately 400 feet above Russell Fiord. We think of no essential differences in origin between the hanging valley above and below sea